nix upgrade-nix: Support new profiles

[edolstra]

Is your feature request related to a problem?

Currently nix upgrade-nix fails on profiles that have been modified by nix profile, typically with a misleading error message like:

error: directory '"/nix/store/46p1z0w9ad605kky62dr53z4h24k2a5r-nix-2.25.3/bin/nix"' does not appear to be part of a Nix profile

DeterminateSystems/nix-installer#1362

Proposed solution

If the profile has a manifest.json file, use nix profile to upgrade the Nix package. It's not entirely clear what the behaviour should be, but probably something like:

• If the profile element providing nix is a store path, then upgrade it to the store path returned by getLatestNix().
• Otherwise do the equivalent of nix profile upgrade <element>.

Alternative solutions

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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